We now do the calculation with the aforementioned AL1 model (REF ), including the mixing of the T and M color components.
The doubly-heavy tetraquarks have been already studied by several authors with this model [1]}, [2]}, with the good surprise that a tiny binding is obtained for \(cc\bar{u}\bar{d}\) , and, of course, a more pronounced binding for the heavier analogs where one or two \(c\)  quarks are replaced by \(b\) . This is shown in Fig. REF : the curve labeled AL1-NR crosses the threshold th-NR before \(M=m_c\) , if one fixes beforehand \(m=m_q\) . Two remarks here are in order:
